The invention relates generally to data storage media devices, e.g., disk drives and related technologies.
Data storage media, such as disk drives, may comprise one or more magnetic disks on which information may be stored as corresponding magnetic polarities. For example, a series of information bits, e.g., “1010” may be stored on the magnetic media as magnetic transitions corresponding to +1, −1, +1, −1. Conventionally, using what is known as “continuous magnetic media,” there is no strong requirement for the accuracy of the absolute positions of the written data positions. With continuous media, preambles, or training patterns, are written as part of the write operations, to depict the start of a data sector and the start of the data within the sector. In addition, the training patterns provide timing information for read clock synchronization, since the training patterns are written at the same time as the data using a fixed frequency write clock. As sectors are re-written, the starting points may vary slightly, and thus, read operations must re-synch at the start of each sector to ensure alignment of the read operation to the start of the data as well as the timing of the data.
With continuous magnetic media, the system reads a given sector by locating the associated training pattern and synchronizing a variable frequency read clock to the frequency and phase of the pattern as read from the medium. The synchronizing of the read clock is required to overcome differences in disk speed between the read and write operations, differences in fly height, and so forth. At the start of the sector the read clock is brought into frequency and phase synchronization with the recorded training pattern by a read channel digital phase lock loop. After the read clock is synchronized to the training pattern data, the read clock is synchronous with the data, which was recorded at the same time using the same fixed-frequency write clock.
Bit patterned media (“BPM”), on the other hand, is a relatively new technique used in magnetic data storage that provides patterns of magnetic regions (e.g., “dots” or “islands”) within non-magnetic material. In contrast to conventional continuous magnetic media, for efficient use of BPM capacity, write operations to BPM must be aligned such that write current transitions are synchronized with the patterns of dots. Synchronization is also required for reading the magnetic states of the dots.
One solution to synchronize a write clock is described in above-mentioned U.S. patent application Ser. No. 12/267,305. As described therein, Phase-Locked Loop (PLL) synchronization fields may be interspersed between servo sectors to provide media-referenced timing for write operations. For example, a write clock controller may be used to control the phase and frequency of a write clock based on the PLL fields, and the updated write clock timing may then be coasted during the interval between PLL fields.
In particular, a system using interspersed PLL fields to attain write synchronization generally needs to periodically read PLL fields during a write operation. Because the reader and writer are typically separated by a distance greater than the PLL field length, the writer is generally over a data field when the reader is over a PLL field. One technique that may be used in this situation is to simply continue to write data while the PLL field is read, requiring a read-while-write operation. This results, however, in significant effects of write coupling, where radiated and reactively coupled artifacts of the write signal corrupt the sensing of the signal to be read. While techniques may be employed to minimize the coupling enough to enable a relatively clean read of a PLL field, the techniques are complex. Conversely, as described in the above-mentioned U.S. patent application Ser. No. 12/267,305, a second approach is to disable writing as the reader approaches the PLL field, and to resume writing after the PLL field has been read. This second method incurs the format overhead of allowing for lengthy read-to-write mode operation transitions, where write current is turned off and then restored to a programmed write current magnitude (often referred to as “ramp up”).